Fire protection nozzles are used to discharge water, with or without additives, in a relatively fine spray, which is generally referred to in the industry as mist. Nozzles with an inwardly curvilinear inlet section and, in particular, nozzles with an inwardly convex section for which normals to the tangent lines at neighboring points on the curve tend to diverge, are utilized for the purpose of discharging a fire-retardant liquid.
Various types of nozzles discharging a fine water spray have long been used in fire protection systems. Although often not described as such at the time, perforated diffuser sprinklers such as described in Parmalee U.S. Pat. No. 6,257 discharged water in a fine spray by nature of the diffuser holes being in the order of 0.06 inch in diameter. Other examples of fine spray nozzle designs intended for use in fire protection system applications are described in Lewis U.S. Pat. No. 2,310,798, which is based on the use of impinging jets to create a "cloud" of spray, as well as Loepsinger U.S. Pat. No. 2,361,144 and Papavergos U.S. Pat. No. 4,989,675, which are based on establishing a gas-water mixture to create an atomized spray. Further techniques for delivering fine spray for fire suppression purposes include: using an array of nozzles originally designed for fine oil mist atomizing, e.g. in oil burner applications, and using nozzles with an internal fixed scroll, or a whirling device, e.g. as described in PCT Publication No. WO 92/20454.
The mechanism(s) by which fine spray (water mist) acts to control, suppress or extinguish a fire can be a complex combination of two or more of the following factors, depending on the operating concept of the individual nozzle, the size of the orifice(s), the operating pressure and flow rate:
1. Heat extraction from the fire as water is converted into vapor
The amount of evaporation and hence heat withdrawn from the fire (i.e., cooling of the fuel) is a function of surface area of water droplets applied, for a given volume. Reducing droplet size increases surface area and increases the cooling effect of a given volumetric flow rate of water.
2. Reduced oxygen levels as the vapor displaces oxygen near the seat of the fire
When water converts to vapor, it expands by a factor of about 1650 times, displacing and diluting oxygen, thereby blocking the access of oxygen to the fuel. Arsonist fires in enclosures are, therefore, the easiest for water mist systems to extinguish because of the virtually instantaneous vaporization which can occur due to the relatively high level of heat present at nozzle operation, even with fast response release elements.
3. Deluging of the protected area
Small water droplets are extremely light, and tend to remain suspended with the slightest air currents. This results in a "mist" that tends to distribute itself throughout an enclosure, outside of the direct spray range of an individual nozzle. Fine water droplets are, therefore, more likely to be drawn into the seat of the fire, further enhancing the effectiveness of the system by chemically inhibiting the combustion radicals. This three-dimensional effect of the expanding mist also acts to cool the gases and other fuels in the area, blocking the transfer of radiant heat to adjacent combustibles, as well as, pre-wetting them.
4. Direct impingement wetting and cooling of combustibles
In addition to the pre-wetting and cooling of the flames by vaporizing water droplets, fire extinguishment by direct contact of the water droplets with the burning fuel to prevent further generation of the combustible vapors is one of the modes of fire extinguishment normally associated with traditional sprinklers having orifice diameters most often of about 0.44 inch or larger. However, with a fast response release mechanism, high momentum mist can be effective in this mode during the early development stage of exposed fires.
Generally speaking, the sizes of the orifices used in water mist nozzles are in the order of 0.06 inch in diameter or less, with the orifice diameter becoming smaller as the flowing pressure is increased, in order to restrict the flow to a reasonable value. For example, nozzle assemblies made up of fine oil mist-type sprayers generally have orifice diameters in the order of 0.02 inch or smaller and are operated at pressures of about 1,000 psig or higher. As compared to traditional sprinklers with orifice diameters most often of about 0.44 inch or larger, water mist nozzles with orifice diameters of about 0.06 inch or smaller require the use of fine inlet mesh strainers to prevent clogging due to debris in the water supply, while nozzles with orifice diameters of 0.02 inch or smaller are considered to be excessively susceptible to clogging by either debris or mineral deposits in the water supply or corrosive atmospheres like that associated with a marine environment. As such, very fine mesh inlet strainers are needed to protect the orifices, the nozzle bodies need to be made of costly corrosion resistant materials and, in addition, the use of deionized water as well as protective exterior caps (which would blow off following nozzle operation), should be considered. Lastly, operation of water pumps at 1,000 psi or higher, especially in marine service, raises questions as to the degree of maintenance required in order to ensure the level of reliability necessary for helping to assure safety of life in a fire situation.
Dual media water mist systems such as the gas-water mixture system described in Papavergos U.S. Pat. No. 4,989,675 tend to have a larger and more acceptable water discharge orifice diameter (in the order of 0.12 inch) and operate at pressures in the order of 45 psig to 75 psig. However, dual media systems have the extra costs and complexity associated with installing two sets of piping to each nozzle, they must be operated as a deluge system (e.g., water is flowed from a number of nozzles at once, to cover a relatively wide area), and a separate source of relatively high flow rate compressed gas must be maintained. The gas source is normally provided by using cylinders of compressed nitrogen at pressures of greater than 2,000 psig, and, because of the fixed volume of gas supply, it is also necessary to make provisions for discharging multiple shots of the water mist, with each shot lasting a few minutes, in the event that the fire re-ignites after the first shot of the mist. This makes the equipment more complex and costly. Lastly, with the dual media system, care must be taken to prevent over-pressurization of a compartment, otherwise structural damage to the compartment might result upon release of the gas-water mixture.
There is also a variety of background information concerning nozzles, with various types of inwardly convex curvilinear inlet sections for which normals (i.e., perpendiculars) to tangents at neighboring points on the curve tend to diverge, that have been used for applications such as discharging: fire-retardant fluids, water for irrigation, rocket fuels, and chemicals used in industrial processes. Prior art illustrating nozzles with various types of inwardly convex curvilinear inlet sections, which are used for discharging fire-retardant liquids, include the following: Gilmore U.S. Pat. No. 488,003; Reed U.S. Pat. No. 781,159; Berna U.S. Pat. No. 1,315,079; Livingston U.S. Pat. No. 3,872,928; Livingston U.S. Pat. No. 3,884,305; Klein U.S. Pat. No. 4,800,961; Polan U.S. Pat. No. 4,991,656 and Simons U.S. Pat. No. 5,195,592.
Prior art nozzles for irrigation applications are described in Varner U.S. Pat. No. 4,228,956 and Drechsel U.S. Pat. No. 4,842,199. A prior art nozzle with inwardly convex inlet sections for use in rocket fuel applications is described in Ledwith U.S. Pat. No. 3,171,248, while prior art nozzles with inwardly convex inlet sections for use in chemical process applications are described in Devillard U.S. Pat. No. 3,130,920 and East U.S. Pat. No. 3,550,864.